as Daz said above, Pumaspeed do engine conversions for the ka - the 1.6 zetec s (fiesta), and the 1.7 zetec s (puma).
You are looking at probably 1.5 - 2k for the 1.6 and 2.5k + for the puma. I know one bloke in the kaklub, who has had a 1.7 conversion done by them. There was a blue ka in performance ford a few months back who had also had it done by another company (cant recall the name) but they took out the front crossmember.
You could always look at Euro Engines, as they do an exchange deal, so you give them your standard, and you can either have a stage 1, 2 or 3 in return. Stage one is about £600, Stage 2 about £900 ish and Stage 3 = £1400. The Stage 3 is a 1400cc version (bored out). A couple of people in the klub have had this, with decent results (90bhp +), but suffers from fuel consumption a touch. Whereas the 1.6 and 1.7 conversions should keep you at 30mpg+ easily.
1400cc = 90bhp ish, 1600cc = 105bhp ish, 1700cc = 125bhp ish.
The RS Turbo lump in a ka has been done a few times, but I am still yet to see a reliable every day car as a result.
Hope that is of some help.....